This career development award proposal aims to define potential mechanisms that predict outcome after traumatic brain injury (TBI), while at the same time providing the applicant with training opportunities within the VA. TBI affects approximately 15% military population. Veterans were subjected to repeated TBI are at increased risk of developing long-term neurodegenerative disorders. Treatment options are limited, mainly because the pathology behind neurological deficits due to TBI is poorly understood. Apolipoprotein E (ApoE) has several alleles, and patients that express the E4 allele have worse outcomes, particularly in the long term after TBI. Despite a wide range of studies centered on the ApoE4 allele, none can be found that investigate the influence of a major ApoE receptor, low density lipoprotein related protein 1 (LRP1), on outcomes after TBI. Nevertheless, evidence suggests that LRP1 can play a role in modulating TBI outcome- -LRP1 removes a large variety of cellular proteins, both from the plasma membrane and in the cytosol, and plays an important role in determining protein expression at the plasma membrane. ApoE4 enters cells by binding to LRP1 and undergoing receptor mediated endocytosis. In contrast to other ApoE alleles (E2, E3), ApoE4 endocytosis and receptor recycling are impaired. In the case of LRP1, impaired recycling is postulated to greatly alter ability of LRP1 to act as a clearance receptor, therefore changing the milieu of proteins expressed on the plasma membrane and so altering cellular response to damage. For this proposal, I am focused on understanding the contribution of LRP1 to TBI pathology in a single cell type, astrocytes. Not only are astrocytes the primary producers of ApoE in the brain, they also play a tremendous role in determining outcome by limiting secondary spread of damage after TBI. My working hypothesis is that critical LRP1 function is disrupted by ApoE4 binding in astrocytes, thereby modulating normal cellular responses and sensitizing the brain to inflammation and cell death after TBI. My research strategy will test this hypothesis while also providing clear training objectives and career opportunities in order to develop independence. Aim 1 tests the hypothesis that loss of astrocyte LRP1 worsens outcome in TBI. Astrocyte-specific LRP1 mice have been generated, and mouse TBI models will test outcome in the acute and chronic outcome. Aim 2 will test the hypothesis that the impact of ApoE4 on TBI outcome depends on LRP1. TBI-like cell culture models will mechanistically test the influence of ApoE4 and LRP1 on cell survival. One new mouse line will be generated by crossing ApoE4 targeted replacement mice into the astrocyte-LRP1 knockout model, and then the effect on outcome after TBI will be tested similar to aim 1. Aim 3 tests the hypothesis that ApoE4 worsens outcome by increasing signaling through the inflammatory tumor necrosis alpha receptor 1 (TNFR1), elevating cytokine stimulated cell death. This aim will be tested mechanistically using cell culture models to test the influence of ApoE4 and LRP1 on TNFR1 plasma membrane expression, recycling, and TNFR1-mediated cell death. My career goal is to become a successful independent investigator within the VA system, eventually obtaining MERIT funding and contributing to better outcomes in Veterans with TBI. I have assembled a team that includes experts in the field of LRP1 biology, neuroscience, and most importantly, TBI. The scientific objectives will prepare me for a career studying the effects of TBI in mouse models, while the career objectives will improve scientific critical thinking and provide opportunities for networking and presentation of my results. These will be achieved via biannual evaluation by the advisory committee, attendance at national conferences, attending journal club, improving grantsmanship, and publishing manuscripts resulting from the proposed work. My co- mentors will also provide training related to proper direction of a successful independent research lab. Altogether, I expect that the activities detailed in this proposal will provide an exceptional environment from which to start an independent career.